Superbug spreads in Delhi sewers
* 14:55 07 April 2011 by Debora MacKenzie
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A gene that makes bacteria invulnerable to many antibiotics, including drugs of last resort, has just got a lot scarier. It was thought to pose a threat mainly in hospitals, but it has now been found in sewers and drinking water in India's capital, New Delhi.
And it is likely to be much more widespread than that. The gene has already invaded germs that cause cholera, dysentery and other major infections, and can jump readily among bacteria in the conditions of India's monsoon season.
Surveys are desperately needed to find out where it has gone and what bacteria carry it, say researchers. But political backlash in India has made it hard for scientists based in the country to do so – for instance, the water samples for the current study were collected by a journalist for the British TV programme Channel 4 News, not academic researchers.
The NDM-1 gene codes for an enzyme that destroys all but one antibiotic in a class called beta-lactams. These include the carbapenems, some of which still work against other antibiotic-resistant bacteria and are therefore used as last-ditch treatments – but they won't work against NDM-1.
The gene was discovered in 2009 at the University of Cardiff, UK, in a Swede who had had medical treatment in India. Last August the same team found it in 37 more people in the UK who had had surgery in India, Pakistan or Bangladesh.
Antibiotic-resistant bacteria tend to emerge in hospitals, where the drugs are heavily used, so the discovery was widely reported as a threat to western "medical tourists" who get healthcare in India. Now, however, the Cardiff team reports that NDM-1 is in bacteria from 2 per cent of tap water samples and 30 per cent of street puddles from across New Delhi. Because DNA in the samples had degraded, the researchers say the true prevalence is probably much higher.
It is also unlikely to be in New Delhi alone. "We know it is in patients in Mumbai and Chennai, and I have good information that it is in Calcutta," says Tim Walsh of the Cardiff team, now at the University of Queensland in Brisbane, Australia. People in Pakistan and Bangladesh also had bacteria with the gene, he says. "It is probably widespread in the environment across south Asia."
"If a woman gets a urinary tract infection with NDM-1, it is untreatable," he says, because the only two antibiotics that defeat NDM-1 don't affect the urinary tract. GlaxoSmithKline has a drug that defeats NDM-1 in development but it won't be ready for several years, "and in several years we'll need 10 drugs that work against it, not one", adds Walsh.
Similar antibiotic-resistance genes mostly stay in hospitals, says Mark Toleman at the University of Cardiff. But NDM-1 escaped on a plasmid – a small mobile chunk of DNA – carried by the common gut bacterium Escherichia coli, which is shed in faeces.
The plasmid also carries a toxin that forces bacteria to acquire it, plus several other antibiotic-resistance genes. The team found that a wide range of bacteria, including important human pathogens such as cholera and Shigella, readily pick up the plasmid at 30 ºC, a common temperature during the Indian monsoon season. To make matters worse, sewers also flood during the monsoon.
Clean water and sanitation would help to limit human exposure to the bacteria, say Walsh and Toleman.
Research to track where the gene has gone would also help. But there was a political uproar in India after the team's August paper, because reports of NDM-1 were said to threaten the medical tourism industry. "The Indian scientists who worked with us on our last paper were told bluntly not to work on NDM-1 again," says Walsh.
Mohammed Shahid of Aligarh Muslim University, India, is looking for NDM-1 in faecal and environmental samples nevertheless. "The situation is not miserable," he says, but the threat of international spread of the plasmid "is real and should not be ignored".
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